Functional Groups in Organic Chemistry: Complete List

A functional group is a specific atom or group of atoms within an organic molecule that gives the molecule its characteristic chemical behaviour. The –OH group makes alcohols behave like alcohols, whether the rest of the molecule is tiny (methanol) or long (decanol). Learn the functional groups and you can predict how thousands of organic compounds will react without memorising each one.

Functional groups matter because they organise the whole of organic chemistry. Compounds sharing a group form a homologous series with similar reactions and gradually changing physical properties. From the ethanol in sanitisers to the carboxylic acids in vinegar, everyday chemistry is functional-group chemistry wearing different carbon chains.

Key Concepts

• Functional group: the reactive part of the molecule; the hydrocarbon chain attached to it is comparatively inert.
• Homologous series: a family with the same functional group where each member differs from the next by a –CH2– unit (methanol, ethanol, propanol, …).
• Hydroxyl –OH: alcohols (ethanol, CH3CH2OH). Carboxyl –COOH: carboxylic acids (ethanoic acid, CH3COOH).
• Aldehyde –CHO: (ethanal, CH3CHO). Ketone >C=O: (propanone, CH3COCH3).
• Amino –NH2: amines. Halo –X (F, Cl, Br, I): haloalkanes.
• Alkene C=C: (ethene). Alkyne C≡C: (ethyne). Ester –COO–: (ethyl ethanoate).
• IUPAC naming: the functional group usually supplies the suffix: –ol for alcohols, –al for aldehydes, –one for ketones, –oic acid for carboxylic acids.

Formulas

Each homologous series follows a general formula (for open-chain compounds):

• Alkanes: CnH2n+2 (no functional group — the reference family)
• Alkenes: CnH2n (functional group C=C)
• Alkynes: CnH2n−2 (functional group C≡C)
• Alcohols: CnH2n+2O (functional group –OH)

Limits: these simple formulae assume straight, saturated-or-singly-unsaturated open chains. Rings and multiple functional groups change the hydrogen count, so always verify against the drawn structure.

Worked Example

Write the molecular formula and molar mass of butan-1-ol, and check it against the alcohol general formula.

1. "Butan" means a 4-carbon chain; "-1-ol" puts the –OH on carbon 1: CH3CH2CH2CH2OH.
2. Count atoms: C = 4, H = 10, O = 1, giving C4H10O.
3. Check with CnH2n+2O for n = 4: H = 2(4) + 2 = 10. It matches.
4. Molar mass = (4 × 12) + (10 × 1) + 16 = 48 + 10 + 16 = 74 g/mol.
5. Note: ethoxyethane (CH3CH2OCH2CH3) also has formula C4H10O — same atoms, different functional group (ether), different properties. This is functional-group isomerism.

Common Misconceptions

• "The –OH in ethanol is the same as the OH in sodium hydroxide." In ethanol the –OH is covalently bonded and the molecule is neutral; in NaOH it is the free hydroxide ion OH−. Ethanol is not a base and does not turn litmus blue.
• "Same molecular formula means same compound." Ethanol and dimethyl ether share C2H6O but one is a liquid alcohol and the other a gas — the functional group, not the formula, decides behaviour.
• "Functional groups only occur once per molecule." Many molecules carry several — citric acid has three –COOH groups and one –OH — and each contributes its chemistry.

Real-World Applications

• Sanitisers and fuels: ethanol's –OH group makes it mix with water and burn cleanly.
• Vinegar: ethanoic acid's –COOH group gives the sour taste and preservative power.
• Soaps and detergents: a carboxylate head (–COO−) attached to a long hydrocarbon tail lifts grease into water.
• Medicines: drug designers add or modify functional groups to tune how a molecule dissolves, binds and breaks down in the body.

Spot the functional group first and the molecule's chemistry follows: –OH, –COOH, –CHO, >C=O, –NH2 and the rest each bring their own reactions and naming suffixes. Practise converting names to structures and checking them against the general formulae, and organic chemistry turns from memorisation into pattern recognition.